Welcome to my corner of the web!
My name is Mihai and I work on the Firefox developer tools.
When it comes to web development, I like both server-side and
client-side work. I am mainly interested in web browsers, web
standards and related technologies.
24 March 2014, 10:01
This is just a quick note to the blog readers about a new page on
How to customize the Firefox Web Console output. The API
presented is fully available in the Aurora channel,
and partially available in the beta release
I hope add-on authors interested to extend the developer tools will
find the documentation useful.
12 March 2014, 04:12
We are really close to the next Firefox release, which will happen
next week. This is a rundown of Web
Console changes in current Firefox release channels.
Here is a really nice introduction video for the Web Console, made
by Will Bamberg:
You can learn more
about the Web Console on Mozilla's Developer Network web site.
Stable release (Firefox 27)
Added page reflow logging. Enable this in the "CSS > Log" menu
You can use the
-jsconsole command line option when
you start Firefox to automatically start the Browser
Added split console: you can press Escape in any tool to
quickly open the console (bug
Added support for
console.exception() as an alias for the
console.error() method (bug
currently selected stackframe in the JS Debugger
Added option to toggle the display of message timestamps in the
console output (bug
Added a dark theme for the console output.
CSS warnings disabled by default (bug
Added the console
API to Web Workers (bug
620935). Now you can log messages to the Web Console from Web
Pretty output for objects in the console output. You can now see
previews of object properties, array elements, DOM elements, node
lists, and more (bug
Compact object previews are also used in the object inspector,
which is also used by the JS Debugger.
The changes made in bug 843004 also include new APIs that allow
custom message output to the Web Console. You can customize output
for different object types as well.
Nicer output for
console.trace() - inline display of
stack frames with syntax highlighting (bug
Now the input line is focused when you click anywhere in the
console output (bug
cd() support. Now you can execute code in
cd(someWindowObject) switch the JS evaluation scope to
the given global window object - you can get it from iframes using
cd('.selector iframe') - for ease of use you can
specify a CSS selector that matches the iframe you want to use as
cd(iframeDOMElement) - you can also point to the
<iframe> DOM element.
cd() with no arguments to reset eval scope to
the top window.
cd() is also available in Scratchpad.
Rich previews for objects (bug
584733): syntax highlighting, you can click on property
values/array elements to inspect those objects directly, DOM node
lists and more.
DOM elements are now highlighted in the page when you hover the DOM
element in the console output (bug
757866). An inspector icon is also included - click it to jump
to see the DOM element in the markup view.
Added support for pretty output of objects in the Browser Console
as well (bug
Network requests are now logged when you connect to Firefox OS (bug
917227). This affects both the Web Console and Network
Autocomplete for array members, so now you can type
myArray.foo to get suggestions (bug
Ctrl/Cmd-Shift-K no longer toggles the Web Console. To
close the developer tools use Ctrl/Cmd-Shift-I or
F12. Ctrl/Cmd-Shift-K is now used to focus
the Web Console JS input if it is not already focused. See
Rob's blog post for details and bug
612253 for the development notes.
922161). Set devtools.chrome.enabled to
true in about:config to get
back the input.
Big thanks to all of the contributors who made patches to fix and
improve the Web Console.
Next week we will start work for Firefox 31. If you are a developer
I recommend you to use aurora or nightly builds to get the best of
29 May 2013, 09:21
Today's Firefox Nightly
(May 29, 2013) no longer has the Error Console enabled by default.
It has been replaced by the Browser Console which shows all of the
same errors, warnings and messages as the Error Console and more:
network logging and
window.console API logging from
all content and chrome windows - including extensions! It also has
a better UI, you can filter messages, evaluate JS with chrome
privileges and inspect objects.
If you find missing messages, please file bugs as soon as possible in
the "Developer Tools: Console" component. Thank you!
The Browser Console shares the same code with the Web Console. This
means you will get all of the improvements we are working on - we
are gearing up for a console output UI
If you want to enable the Error Console you can change the
devtools.errorconsole.enabled option to
true from about:config.
Pro tip: in your Firefox chrome code (including
extensions) you can do:
The above code will output to the Browser Console if you have it
open - you can inspect objects, print stack traces
console.trace()) and do a bit of timing
Update: these changes should have no impact in
Thunderbird, XULRunner or any other applications. Currently the
changes are limited to hiding the Error Console menu item from
Firefox by default, and the Ctrl/Cmd-Shift-J keyboard
shortcut now opens the Browser Console instead of the Error
9 April 2013, 14:12
Today we just landed three new Web Console features you can play
with in tomorrow's Firefox
On a related note, Victor Porof also landed the new network monitor
which gives you a very nice UI for working with the log of network
requests and responses. The new tool uses the
Web Console actors we worked on for making the Web Console
remotable - both tools share the same network logging mechanism.
Whenever you are debugging a script in the page you can now
sure you select in which stackframe you want to evaluate your code
- just switch to the desired frame in the Debugger view. Until
today the Web Console locked up while debugging pages. For
technical details, see bug
This feature required a number of important changes. The Web
Console now uses only the
debugger API to access the content JS objects. With these
changes we are fixing a number of bugs reports by users.
New object inspector
The old object inspector popup has been replaced with the same
variables view from the debugger, allowing us to fix long standing
issues with the old inspector. This is not just pretty UI: you can
now filter/search through the properties by name and value, edit
property name and value, delete properties, and more. You can also
variables view: when you edit a value you can just write
$("foo") or whatever you wish.
With this change object inspection works slightly different: you
will notice that many of the methods and properties of DOM objects
show in the prototype of the objects you inspect. The switch to the
The new Browser Console
To enable the Browser Console make sure you set
true in about:config (Firefox restart is needed). Open
the Browser Console from the Web Developer > Browser
Console menu item.
The new Console is meant for browser and addon developers: you can
see all of window.console API calls, from all windows,
all script and style errors/warnings, and all network requests from
all over the browser and from addons.
We plan to replace the Error Console with the Browser Console as
soon as possible: the Error Console provides users with less
capabilities than the Browser Console, and it is not remotable. You
can currently connect to any Firefox Mobile, Firefox OS and Firefox
for desktop instance and see all of the logs in the Browser Console
from the remote instance.
Given the amount of logging happening we plan to support better
ways of filtering output. For example, we would like to allow
developers to filter messages by addon.
The screenshot shows the Browser Console with the new object
What is next
All of the three features are the outcome of a lot of work from the
entire team. Still, I want to thank Panagiotis Astithas and Jim Blandy for the
debugger-related work and reviews, and Victor for the variables
Download a Firefox nightly
build, play with the new features until you break them. :)
Please let us know if you have any comments and report any bugs you find!
10 October 2012, 08:49
Here is a summary of the improvements we have made to the Web
Console in the latest versions of Firefox. In the new
Firefox 16 release we have added the ability to
Security Policy warnings and errors. We are also highlighting
network requests that happen over HTTP on HTTPS pages (and
vice-versa), and we did more output performance fine-tuning.
In the new Firefox Beta we have moved the Web
Console UI into its own
<iframe> - which gives
us better flexibility for upcoming work we will do. We changed the
UI to match the developer tools theme and we made objects you pass
console.log() inspectable. For example, if you call
console.log(document) you can click
also received a number of improvements. Last, but not least, now
you can zoom in/out the text in the Web Console - just press
Ctrl-- / Ctrl-+. The default font size should
also match your system's font size settings.
In the new Firefox Aurora release we landed a big
chunk of work that makes the Web Console remotable - we now make
use of the Remote
Debugging Protocol. This means you will be soon able to connect
Web Console instances to your B2G, Fennec or other Firefox for
desktop browser instances. We currently lack UI to allow you to do
this, but we are working on it. This work also paves the way to a
Global Console that could replace the Error Console, some
At the end of September the developer tools team had a
meetup in London. There I had the chance to
demo the Web Console client connected to a B2G
In the above screenshot you can see the Web Console client running
in a local Firefox for desktop build while connected to a local
build of B2G
Desktop running Gaia. You can see network logging, object
inspection, network request/response information and script errors
coming from B2G.
To learn more about the Web Console remoting capabilities see
768096 and the
wiki page on MDN. To follow the progress
with B2G integration see bug
What is next? We are going to complete work on B2G
integration, improve the way Web Console output works, and we will
probably add a Global Console. We also plan to improve the user
experience of the object inspector and the network panel.
6 June 2012, 03:35
For the Aurora update that's happening this week we have some major
changes under the hood for the Web Console.
Async Web Console
Work on making the Web Console UI async, decoupled from all the
error and network listeners, started in July last year (bug
673148). Even if it took almost one year to get this bug fixed,
I worked on these patches for about 3 months (loosely summing up
the total days of work spent on this specific bug).
Back in those months we strongly pushed for various developer
tools to land and get enabled by default in Firefox. I've been
working a lot on the source editor which was needed for the Style
Editor, the JS debugger and Scratchpad. Work on the Web Console was
on and off.
There was modest to good progress for the Web Console async
work until around September - October when Mozilla's electrolysis
project was re-prioritized. The initial work on the patch started
out with the goal of making the Web Console ready for e10s. When
priorities changed, I went back to source editor work which was
higher priority at that point.
In January - February a really brave and courageous contributor,
Sonny Piers, took the huge patch and rebased it. His efforts were
commendable given the size and complexity of the work that was
going on. Thank you Sonny!
In March I resumed work and I strongly focused on completing
the async patches. Last week the last patch landed in the nightly
builds of Firefox.
What changed? Most of the Web Console was
implemented in a single file,
HUDService.jsm. It had
everything - from UI code to all the error and network listeners
and stuff for the
window.console API. We have now
broken that code into separate scripts with the goal to leave
HUDService.jsm as the script that implements only the
UI. The new
HUDService-content.js script implements
all the listeners, all the "backend stuff". The UI code must no
longer directly access the content window and objects from the
Why? This work allows us to move to the remote
debug protocol and to have the Web Console UI connect to your
Firefox Mobile or B2G device where all the error and network
listeners are instanced. This work paves the way to a remotable Web
The added benefit is that the async-ness had some modest
performance benefits to content scripts (pages) that used the
window.console API - a call to any method no longer
had to wait for the Web Console UI parts to update.
In the future other Firefox components and extensions can build
different UIs on top of the data collected by the
- Focus, focus and focus! Given big projects one must not try to
do everything else.
- Do not underestimate the time it takes to polish working code,
to make it ready for review. I had working code in
September-October, but getting it "done" took quite more.
- Make sure your manager is aware there's a ton of work to do on
your project. There's high temptation to be nice and be helpful and
do a lot of other work in between. ;)
Aggressively split your work into smaller chunks.
- Be lazy - avoid doing work you don't need to do for the given
Thanks go to Rob Campbell, Dave Camp, Felipe Gomez, Ms2ger, Joe
Walker, Sonny Piers and everyone else who contributed to getting
these patches to be ready to land.
Building on top of the async Web Console work we've also made some
really nice output performance improvements (bug
722685). In bug
746869 Boris Zbarsky analyzed the performance issues in our
code and he made a number of valuable suggestions on how we can
make it faster. Thank you Boris!
Our first attempt to make the Web Console output faster has landed
in Firefox. Let's go straight for the numbers:
Opera 12 (post-beta, latest snapshot, with "cutting-edge"
Chromium 18 (beta):
- Simple string: 21 ms
- Interpolation: 11 ms
- Simple string: 66 ms
- Interpolation: 68 ms
Performance in content pages is very good. However, display
performance is actually poor. First run is fast. Subsequent runs
take far more. The web inspector tool UI is frozen for many seconds
when the second and third runs happen. Content process separation
helps a lot. Even if Web Inspector's display is frozen, web pages
continue to run smoothly.
Firefox 13 (without the async patches):
Firefox 15 nightly (with the async patches landed):
Firefox 15 Aurora (with the performance patch landed):
- Simple string: 50 ms
- Interpolated string: 48 ms
- Simple string: 51 ms
- Interpolated string: 48 ms
For comparison: do note that 1000
dump() calls take
around 10-20 ms in Firefox. (
dump() is a dumb method
we use to output messages to STDOUT.)
Having the Web Console open or closed no longer directly impacts
console API calls. Now the UI no longer freezes and results show up
I tried with 5000 calls and we now do better than Opera's Dragonfly
and Chrome's Web Inspector - in terms of UI updates. Still, console
API calls finish faster, for some reason, in those two browsers.
Please do note that I used the simple
test attached to bug
722685 for testing. These numbers are not meant to be
"scientific" or anything like that - they are based on my machine
We will continue to do further work in improving the output
761257). At this point we still need to avoid doing some
unneeded work when a lot of messages end up in the queue to be
displayed. We also need to better balance how often and how many
messages we display during "heavy fire" - during the execution of
content scripts that invoke the console API methods many, many
times for an extended period of time.
We have plans to move the Web Console UI into its own
<iframe>, change the UI to match the other
developer tools theme, add the option to move the UI into a real
window, make a global console that could replace the Error Console
and, obviously, switch to the remote debug protocol so you can use
the Web Console with remote Firefox instances. All this and many
other improvements, of course!
You may wonder "when?" and the answer to that is that all the
improvements will come gradually when we get to implement them.
File bugs, find regressions and let us know what you like and
dislike! Thank you!